OBJECTIVE: To determine whether demonstable differences exist between morphologically indistinguishable trophozoites of the genus Giardia. Organisms to be studied have been isolated from man and a number of lower animals, and maintained in culture. All of the organisms are capable of growth in a culture medium which contains Hanks' solution, inactivated human serum, Phytone, and tissue culture medium NCTC-135. METHOD: The antigenic nature of these organisms will be studied and compared. Antigens will consist of washed, sonicated trophozoites. Rabbits, the source of antiserum, will receive mixtures of antigen and Freund adjuvant. The optimum immunization schedule will be determined. The number of antigens demonstrable will be determined using gel diffusion and immunoelectrophoresis techniques. Comparative immunoelectrophoresis will also be employed to determine whether or not different trophozoite preparations are identical.